inspect.docs: Document that Signature and Parameter are now picklable (issue #20726)

Doc/library/inspect.rst

@@ -462,6 +462,9 @@ function.
    Signature objects are *immutable*.  Use :meth:`Signature.replace` to make a
    modified copy.
 
+   .. versionchanged:: 3.5
+      Signature objects are picklable.
+
    .. attribute:: Signature.empty
 
       A special class-level marker to specify absence of a return annotation.
@@ -526,6 +529,9 @@ function.
    Parameter objects are *immutable*.  Instead of modifying a Parameter object,
    you can use :meth:`Parameter.replace` to create a modified copy.
 
+   .. versionchanged:: 3.5
+      Parameter objects are picklable.
+
    .. attribute:: Parameter.empty
 
       A special class-level marker to specify absence of default values and